IMPALA-4849: IllegalStateException from rewritten CASE expr

In SelectList.reset(), we call reset() on each select list item's
expr. reset() is supposed to remove implicit casts, by returning
the reset expr with implicit cast exprs removed from the tree.

Previously SelectList.reset() ignored the return value of the calls
to Expr.reset(), meaning that if the top-most expr of the select list
item is an implicit cast, it won't actually get removed, which causes
problems with analysis since implicit casts are always treated as
pre-analyzed.

The solution is to set the select list item's exprs to the return
value of reset().

Testing:
- Added a regression test to exprs.test

Change-Id: I16ff88716b185e1d72d2bc603a42bd06c60ec18e
Reviewed-on: http://gerrit.cloudera.org:8080/5917
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: Impala Public Jenkins

testdata/workloads/functional-query/queries/QueryTest/exprs.test

@@ -2692,3 +2692,18 @@ where id = case when false then 0 when 1 = 1 then 1 else 2 end
 ---- TYPES
 INT,TINYINT
 ====
+---- QUERY
+# IMPALA-4849: verify that DISTINCT with a CASE that is rewritten analyzes correctly.
+select distinct case when true then id else 0 end from functional.alltypestiny
+---- RESULTS
+0
+1
+2
+3
+4
+5
+6
+7
+---- TYPES
+INT
+====